It is generally known that oxidizing inorganic and organic species in dilute aqueous solutions by electrolysis is nearly impossible to accomplish because of the poor mobility of these species in such aqueous solutions to reach the anodic site where oxidation takes place.
An example of this technology, which is not limited in scope to this example, is the production of halous acids from dilute concentrations of their corresponding halide salts. Such halous acids are particularly useful as oxidizing agents. It is known that electrolyzing dilute halide salts to form their respective halous acid solutions is difficult to accomplish without forming other fully oxidized species such as the halates, which have little oxidizing effect. In order to solve this problem, a number of anodic systems utilizing noble metal catalysts have been developed to prevent the halate ion side reactions from taking place. Even with the latest dimensionally stable anodes, the side reactions will predominate unless the halide concentration in the aqueous solution being electrolyzed exceeds 1500 mg/l. At these elevated concentrations, a small percentage of the halide can be converted to the halous ion.
In actual practice, however, the halide salt concentration is kept between 5000 mg/l and 250,000 mg/l in order to convert a small percentage of the halide directly to the halous solution. At these high salt concentrations, the side reactions that form the undesired halate ions are significantly reduced or eliminated. However, the high concentration of residual salts causes additional problems. Concentrations of greater than 1500 mg/l of halide ions cause corrosion of various bimetal connections found in water plumbing systems, such as water distributors, cooling towers, and swimming pools, as well as any other aqueous process equipment. Thus, electrolysis has not proven to be a reasonable method for the production of halous acids, because the conversion rates are small, and the residual salts are harmful to the distribution systems.
Therefore, in order to control microbiological contamination in these aqueous systems, highly concentrated and potentially dangerous halous solutions are dosed into the dilute streams of water to maintain an adequate concentration of the halous acid needed to control or destroy the offending or undesirable microbes. These microbes can be as simple as pseudomonas and coliform in drinking water or can be viruses and gram positive organisms found in cooling towers, such as legionella. The concentration of these halous oxidizers is controlled in these systems by dosing in the chemical so that a permanent level of the oxidizer is maintained, sufficient to kill the target organism(s) by oxidation or to penetrate colonies or large organisms to disrupt the cell mechanism that causes growth. Contact time, concentration, and pH affect the activity or efficacy of the resultant solution.
Most halous solutions used in present systems are shipped to the point of use in the halite form to prevent autodecomposition of the halous acids back to their salts during transportation. Therefore, most of these solutions are shipped with an excess of caustic in order to render a very high pH for the solution, which ensures that they are active when the potential user requires their use. The user relies on the buffering capacity of the treated water to lower the pH of the halite solutions to a point where the halous acid forms and the solution becomes active. If the treated solution does not have the proper acidity, either too high or too low, required to produce the desired halous acid concentration by dosing, however, either acid or caustic must be added to maintain the activity level and effective half life of the halous acid.
All water as it is received in nature has various levels of salts, hardness ions, alkalinity, and pH which make it unfit for consumption or further commercial and industrial uses. Therefore, mechanical and chemical treatment, such as filtration and chlorination, must be performed on it to render it potable or fit for further use. Indeed, the Public Drinking Water Act and the World Health Organization require that potable water contain less than 500 mg/l of dissolved solids. As stated previously, it is impractical with the present technology to electrolyze water which contains less than 1500 mg/l of dissolved salt to form halous acids, such as hypochlorous acid. Therefore, potable water is impractical to electrolyze directly because of its low salt content. To make the water easy to electrolyze, salt must be added which then makes the water non-potable.
The dosing of municipal, industrial and commercial water systems is a major undertaking, requiring the shipment of enormous quantities of halogen solutions. It is estimated that in excess of 20 million tons of halogen solutions are shipped annually, in the United States and Canada alone.
When the contrary problem arises, where the solution to be treated contains too high a level of oxidizing substances or oxygen, chemical reducing agents are added in excess to reduce the oxidation problem to levels that present satisfactory operating conditions. These reducing agents can be as simple as carbon for strong oxidizers such as chlorine, or they can be strong reducing agents such as hydrazine which are dangerous to ship, handle, and dispense in accurate dosages.
The problem of the prior art electrolytic systems is that dilute salt solutions have low conductivity which results in low mobility of the reactive species to reach the appropriate site on the anode or cathode where the oxidation or reduction reaction can take place. The oxidation or reduction of the reactive species occurs when the free radical hydroxyl or free radical hydrogen, produced by the splitting of the water molecule at the cathode or anode, respectively, is contacted by the reactive species. As used herein, it is intended that the term "free radical hydroxyl" be synonymous with other terms used in the art for this electrolytic ion, such as previously used terms "nascent oxygen," "molecular oxygen" and "singlet oxygen." Similarly, the term "free radical hydrogen" is intended to be synonymous with other terms used in the art, such as "nascent hydrogen" and "singlet hydrogen."
The starvation of such reactive ions species as halide salts to the anode or cathode is a phenomena known in the art as polarization, and it results in the excess generation of free radical hydroxyl or free radical hydrogen which continues to oxidize or reduce the reactive ion species into a nonusable halate solution. An additional significant problem associated with electrolytic conversion of dilute halide solutions, as well as other dilute salt solutions, arises from the fact that the surface area of the anode or cathode is limited so that intimate contact between the species to be oxidized or reduced and the free radical hydroxyl or free radical hydrogen does not occur. Hence, very poor conversion of the species to be oxidized or reduced can be achieved. These two major problems existing with prior electrolytic methods have not been overcome to date.
Various mechanisms have been tried for mixing, and porous and packed bed electrodes have been tried, for the oxidation of halides to halous acids, without success to date. Indeed, the electrolytic industry has relied on noble metal oxides attached to substrates of titanium and its analogs to form desired semiconductor junctions at which water can be split and oxidation can take place. Porous electrodes help to solve the problem of large electrode surface area, but they do not resolve the problem of ionic mobility in dilute solutions.
There are many known electrochemical processes using resin and/or membranes in combination for many varied purposes, including the electro-demineralization of water both by empty and filled-cell electrodialysis, the electrodialytic concentration of soluble metals from dilute solutions onto the resins in the electrochemical apparatus, and the production of chlorine from concentrated brine in membrane chlor alkali cells. For example, in U.S. Pat. Nos. 4,299,675 and 4,356,068, ion selective membranes are used as an immobile electrolyte and the electrodes are bonded to the membrane to reduce ionic resistivity. Also of interest are U.S. Pat. Nos. 4,369,103 and 4,636,286. However, no prior art system has been effective in electrolytically oxidizing, or reducing, reactive species in dilute solutions having salt concentrations less than 1500 mg/l, especially in oxidizing dilute halide solutions to halous solutions.